python报错：'OpenpyxlWriter' object is not subscriptable不知道在哪里有错误

'OpenpyxlWriter' object is not subscriptable 运行出现这个错误，希望各位能帮解决一下。感谢！
错误代码：
Traceback (most recent call last):
File "E:/lyw/pythonProject/geography/statistical table.py", line 442, in
tables = Tables()
File "E:/lyw/pythonProject/geography/statistical table.py", line 17, in init
self.dataprocess()
File "E:/lyw/pythonProject/geography/statistical table.py", line 27, in dataprocess
split_data1 = self.data["年月日"].astype(str).str.split('.', expand=True)
TypeError: 'OpenpyxlWriter' object is not subscriptable

源代码：

```python
class Tables(QMainWindow, Ui_Dialog):
    def __init__(self):
        super().__init__()
        self.ui = Ui_Dialog()
        self.setupUi(self)
        self.dataprocess()
        self.zhenji.clicked.connect(self.zhenjishaixuan)
        self.nianfen.clicked.connect(self.nianfen_0)
        self.queding.clicked.connect(self.save)
        #self.longitude()

    def dataprocess(self):
        self.data = pd.ExcelWriter('区划图强震目录.xlsx')
        self.outputfile=pd.ExcelWriter(r"E:\lyw\text.xlsx")# 年份
        # 对指定列进行分割
        split_data1 = self.data["年月日"].astype(str).str.split('.', expand=True)
        print(split_data1)
        # 修改分割后的字段名称
        split_data1.columns = ['D_' + str(i) for i in split_data1.columns]
        self.data = self.data.join(split_data1)

self.data["年月日"].astype(str).str是个OpenpyxlWriter对象，不是str类型，不能split
你可以先强制转str试试

可以将数据发出来瞅瞅嘛